Hydrostatic torque converters, generally speaking, are intended to be disposed between an engine and a load and have, as is the case with all torque converters, an input element, generally a shaft connected with the engine, and an output element which is usually also a shaft and is connected with the load. Between the input element and the output element, means is provided for adjusting the transmission ratio and torque transmission between the shafts.
In the case of a hydrostatic torque converter, as opposed to a hydrodynamic torque converter, the input shaft generally drives a variable-displacement hydrostatic pump (e.g. an axial-piston pump whose displacement per revolution is adjusted by tilting a control plate, disk, or cylinder drum), the hydrostatic pump being hydraulically connected to a hydrostatic motor which can be of the fixed displacement axial piston type. The hydrostatic pump and hydrostatic motor together form a hydrostatic transmission and the motor shaft is generally connected to the input side of a stepped gear transmission which is shiftable under load. In a stepped gear transmission shiftable under load of this type, a driving connection is effected between the input shaft of the gear transmission and the output shaft thereof, the latter constituting the output shaft of the torque converter, by engaging selectively one of a plurality of clutches to establish effective gear trains between the shafts.
The process of switching over between the steps of such a transmission consists in deactuating one of the clutches of a previously effective gear train and engaging or actuating a clutch of another gear train which is to take over the driving operation.
It is a common practice in such torque converters to control the hydrostatic transmission of the apparatus by a servopiston which can be connected to the stroke or displacement controlling element of the pump, in order to compensate for the jump in speed which would otherwise result from the shifting of steps in the gear transmission.
In other words, a servo- or regulating piston is provided in a power controller and is connected to the control element of the hydraulic pump, this piston being displaced in response to a control valve in a hydraulic control circuit. The valve, in turn, is shifted in response to signals applied to the clutches to be switched.
In a conventional hydrostatic torque converter of this type, described, for example, in the open German application (Offenlegungsschrift) DT-AS No. 2,237,595, the variable displacement pump of the hydrostatic component of the torque converter follows the gear-transmission shifting under load to effect a speed equalization or compensation for the step in transmission ratios of the gear transmission.
In this system, the pump has its stroke or displacement controlling element connected to a servopiston which responds to a speed sensor via the switching valve mentioned previously. In this arrangement, the clutch speeds at each instant during the stepping of the gear transmission are detected and serve as inputs for controlling the valve which, in turn, operates the servomechanism for controlling the displacement of the stroke control element of the pump.
The speed signal operates independently of the load transfer between the generation of the switch command to synchronization of the clutching.
When this type of torque converter is used, especially in a series assembly with others, the positive or negative overlap of the load variation from disengaged to engaged clutch does not always result in a shock-free transition.
It has been proposed (see published German Application-Auslegeschrift-DT-AS No. 2,237,595) to use the clutch switching pressure directly at the instant of switchover of the gear transmission to operate the hydrostatic component of the transmission. This control system, however, causes the variable displacement pump to be actuated immediately upon initiation of the transition ratio switchover of the gear transmission so that, during the forced actuation phase, corresponding to positive switchover lap of the clutches, the compensating adjustment of the hydrostatic component has already commenced or been completed. As a consequence, the adjustment of the hydrostatic component is effective before the selected step of the mechanical transmission has completely taken over the load and a shock is imparted to the load. When the load is the driving wheels of a vehicle, this system results in a jerky gear change of the vehicle operation.
The term "forced actuation" as it refers to a positive overalp of the operations of the clutches of the mechanical transmission is discussed in greater detail below. However, for better understanding of the prior-art systems, it should be appreciated that this phase in the operation of the mechanical transmission is the phase in which both gear trains are coupled simultaneously between the input shaft and the output shaft of the gear transmission. In a load-shiftable transmission, i.e. a transmission shiftable under load, this phase or period corresponds to the interval at which the clutch to be de-energized is still practically fully engaged although hydraulic fluid is in the process of being drained from its actuating element, while hydraulic fluid is in the process of being fed to the actuating element of the clutch of the gear train intended to take over the load. During this interval, both clutches are at least partially engaged and the gear system between the input and output shafts of the gear transmission, because of the different transmission ratios, results in slip in one or both clutches.
Another system has been described in German published application (Auslegeschrift) DT-AS No. 1,817,764 in which the hydrostatic component of the torque converter is actuated to follow the shifting of gears only upon complete actuation of the clutch to be energized and while the clutch to be de-energized has not been fully disengaged. Here, too, a jerky movement is imparted to the load and hence to the vehicle.